1. Field of the Invention
The present invention relates to a self-diagnostic apparatus of a vehicle for storing and holding diagnostic data necessary for an analysis of abnormality in a vehicle on-board device.
2. Description of Related Art
Present-day electronization of vehicles is remarkable and devices carried on individual components of a vehicle including an engine are coupled to each other by means of a control computer and perform complicated operations.
In this case, even with an abnormality in the operation of a single on-board device detected, a true cause of the abnormality as related to other on-board devices often cannot be determined unless data indicative of a vehicle state subject to the abnormality (diagnosis data) is collected over a wide range. Also, operation sometimes recovers normality naturally after a temporary abnormal operation and this often becomes a predictive phenomenon of a total fault; but a cause of the phenomenon is very difficult to find out through an inspection conducted after the driver alights from the vehicle.
Accordingly, in JP-A-62-142849 and JP-A-63-90738, a self-diagnostic apparatus has been proposed wherein diagnostic data of individual vehicle components is updated and stored at predetermined intervals of time in a memory which holds the contents of the diagnostic data even when the ignition switch is turned off, and after an abnormality of an on-board device is detected inhibiting, updating of the above-said memory contents (frozen) in order that a cause of the abnormality can be grasped correctly after the driver alights from the vehicle.
Also proposed in JP-A-3-92564 is an apparatus wherein in addition to the diagnostic data, a control program is also stored in the memory in an atempt to grasp a more correct cause of the abnormality.
In any of the aforementioned apparatus' in which diagnostic data is read and stored at a predetermined period of time (every constant time or every constant engine revolution number), however, the diagnostic data changes greatly within a short time during a transient operation of a vehicle or of an engine and diagnostic data which is frozen upon the detection of the device abnormality sometimes fails to correctly indicate a value at the point in time at which the abnormality occurs.
Conventionally, in prevention of this disadvantage, the read interval is always set to be short but this raises a problem in that the RAM capacity necessary for storing the increased amount of data obtained within a constant amount of time increases.